This invention relates to the art of cathode ray tubes and, more particularly, to an improved dynamic focusing circuit therefor.
Whereas the invention will be described with reference to a video display terminal employing the improved focus correction circuitry for a cathode ray tube, it will be appreciated that the invention is not limited thereto and may be used in various applications requiring focus correction for a cathode ray tube.
One concern in the art of cathode ray tubes has been the development of suitable dynamic focusing circuits for automatically compensating for distortion resulting from defocusing as the beam is scanned across the face of the tube, which is typically substantially flat. The distortion encountered is maximum at the extremes of the scan, such as at the top margin and bottom margin in a vertical scan. It has been known in the art that a corrective signal to offset the defocusing function should vary in magnitude as the beam is being scanned, providing the greater correction at the extremes or margin areas. It has been generally accepted that the corrective signal take a form which is generally parabolic in shape and applied during each period or sweep of the electron beam.
Efforts have been made in the art to provide such a substantially parabolic corrective signal. One example is the patent to M. M. Carpenter, Jr. U.S. Pat. No. 2,728,022. In that system corrective voltages are supplied for both the horizontal and vertical sweeps to compensate for the defocusing. As applied to the vertical defocusing compensation, this system synchronizes the correction with the vertical sync signal provided by television receiver circuitry. More specifically, a sine wave generator is employed to develop a sinusoidal signal having a frequency on the order of one-half that of the vertical sync signal. The sine wave signal is then full wave rectified and then inverted to obtain the generally parabolic shaped correction signal. The full wave rectifier employes a tuned circuit including a transformer and is tuned to the frequency of the sync signal. A notable disadvantage in employing a circuit of this nature is that a tuned circuit for low frequencies requires substantial inductance and, hence, weight. Circuits employing LC tuned circuits or wave shapers have found some acceptance with respect to a horizontal deflection circuit which operate at a high frequency on the order of 28 KHz. However, the vertical sweep rate is at a substantially lower level, on the order of 50 Hz, and, hence, would require a massive LC circuit.